This invention relates generally to a process for irradiating a resist layer, and more specifically to an improved process for sloping the profile of openings formed in a resist layer during the manufacture of semiconductor devices.
Lithography, as used in the manufacture of semiconductor devices, is the process of transferring a collection of geometric patterns on a mask, or reticle, to the surface of a semiconductor wafer and is well known. The lithographic mask used can consist, for example, of a quartz substrate with the desired geometric pattern formed in a thin layer of chrome on one face of the substrate. These transferred patterns will form the component parts of the final integrated circuit such as gate electrodes, interconnecting conducting lines, and transistor device terminals. Each of these component parts is formed in a lengthy process sequence including many interspersed lithography steps with one component part being formed at each lithography step.
In a typical semiconductor lithography process, a photosensitive polymer film, or resist, is formed over a silicon substrate, dried, and then exposed to ultraviolet (UV) or other radiation passing through a mask containing a geometric pattern. If the radiation source is light, then the lithography technique is called photolithography. The silicon substrate is then soaked in a developer which develops the image irradiated onto the polymer film. In the case of a positive resist, the exposed areas of the resist are dissolved and removed by the developer. Conversely, in the case of a negative resist the unexposed areas of the resist are dissolved and removed by the developer. For either type of resist the positive or negative image of the mask pattern is formed in the resist remaining on the silicon substrate. In order to permanently form the pattern in the underlying silicon substrate, the substrate is placed in an ambient which etches surface areas not protected by resist patterns. Finally, the resist pattern is removed, for example by a suitable solvent or dry oxidation, leaving the transferred mask pattern formed in the surface of the silicon substrate.
The edges of the openings formed in the developed resist will have a slope which is characteristic of the lithography method used. It is known that this slope can be imparted to the edges of the openings and features formed in the underlying substrate surface during the etching of the unprotected surface areas, and that additionally, this resist slope can be controlled by selecting an appropriate lithography method. Thus, by controlling the slope of the resist, it is possible to control the slope of the features formed in the underlying substrate surface.
Control of the resist slope is advantageous in semiconductor fabrication, for example in the formation of contact openings to be covered or filled with aluminum metallization and interconnecting conductors (interconnect). It is known that aluminum contacts formed in contact openings with steep, or substantially vertical, slopes are prone to stress-related aluminum cracking or poor contact coverage, and that interconnect formed with steep edges reduces the step coverage of overlying layers formed later in the process. As the number of overlying layers increases, the step coverage reduction becomes even more pronounced. For example, in a double-level metal process, if the first-level metal formed has sloped edges which tend to flatten the surface topography, the subsequently deposited second-level metal and dielectric layers will exhibit improved step coverage.
One existing method for controlling the slope of the resist profile is image defocusing which is performed by slightly moving the resist surface out of the image plane of the optical system. This movement will change the aerial image, defined as the spatially dependent radiation-intensity pattern irradiating the resist, and cause a modification of the resist slope. However, there are several problems associated with defocusing. First, the projected image of the pattern on the resist is distorted as a consequence of moving the resist away from the image plane, and this distortion is randomly variable across the full image field such that control of line-widths to be developed in the resist is unreliable and inconsistent. Second, the distorted mask image produces similarly distorted features in the silicon surface that will partially misalign with features formed in other preceding or subsequent lithographic steps in the process. Such misalignment affects semiconductor device reliability and function.
An alternate method to image defocusing is the slope-bake technique in which the resist viscosity is altered following development to cause a flowing of the resist and thus a modification in the slope of the resist edges. There are several disadvantages to this technique: the introduction of a number of extra processing steps, the need for additional dedicated process equipment, the image distortion due to resist flow, and the loss of dimensional control.
Another alternative directly modifies the pattern on the mask by the addition of narrow stripes to the mask pattern spaced a small distance from the pattern edges on the mask which corresponds to features on the silicon substrate in which a slope modification is desired. The addition of these stripes modifies the aerial image on the resist. But there are numerous disadvantages to this method. The addition of many narrow stripes to the already small geometric patterns of the mask is limited by the resolution of the electron beam used to create the mask and increases the complexity and cost of the mask creation process. Also, these stripes are a permanent change in the mask. There is now way to easily and conveniently alter the character of the stripes on the mask in order to vary the resist slope in response to changes in other parts of the process, for example the use of different resist types. Indeed, new masks must be created to introduce resist slope-control into an existing process or to change the degree of slope modification. Further, the mask created for resist slope modification is dedicated to one specific product and cannot be used for slope modification on other products. Additionally, because the stripes added to the mask decrease the proportion of transparent areas on the mask relative to opaque areas, the efficiency of radiation transmission through the mask is decreased for a given desired mask pattern.
Accordingly, a need existed for a lithography method for modifying the slope of the resist profile in a convenient and flexible manner with undistorted and reliable transfer of the mask pattern to the silicon substrate without additional processing steps.
It is therefore an object of this invention to provide an improved method for sloping the resist profile during lithography.
It is a further object of this invention to provide an improved method for sloping the resist profile which does not require a substantial increase in the number of process steps or process equipment.
It is still a further object of this invention to provide an improved method for sloping the resist profile that does not substantially distort the mask pattern to be transferred.
Still another object of this invention is to provide an improved method for sloping the resist profile during lithography that is flexible.